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Journal Articles

Proposal of simulation materials test technique and their constitutive equations for structural tests and analyses simulating severe accident conditions

Hashidate, Ryuta; Kato, Shoichi; Onizawa, Takashi; Wakai, Takashi; Kasahara, Naoto*

Proceedings of ASME 2020 Pressure Vessels and Piping Conference (PVP 2020) (Internet), 9 Pages, 2020/08

Although it is very essential to clarify how the structure collapses under the severe accident conditions, the failure mechanisms in excessive high temperatures are not clarified. However, it is very difficult and expensive to perform structural tests using actual structural materials. Therefore, we propose to use lead alloys instead of actual structural materials. For demonstration of analogy between the failure mechanisms of lead alloys structure at low temperature and those of the actual structures at high temperature, numerical analyses are required. Although the authors proposed inelastic constitutive equations for numerical analyses in 2019, the equations could not successfully express because of large variations observed in the material tests of the lead alloy. In this study, we propose the improved inelastic constitutive equations of the lead alloy on the basis of the material test results used by aged alloy which can stabilized the material characteristic.

Journal Articles

Mitigation of cavitation damage in J-PARC mercury target vessel

Naoe, Takashi; Kinoshita, Hidetaka; Kogawa, Hiroyuki; Wakui, Takashi; Wakai, Eiichi; Haga, Katsuhiro; Takada, Hiroshi

JPS Conference Proceedings (Internet), 28, p.081004_1 - 081004_6, 2020/02

The beam window of the mercury target vessel in J-PARC is severely damaged by the cavitation. The cavitation damage is a crucial factor to limit lifetime of the target because it increases with the beam power. Therefore, mitigating cavitation damage is an important issue to operate the target stably for long time at 1 MW. At J-PARC, to mitigate the cavitation damage: gas microbubbles are injected into mercury for suppressing pressure waves, and double-walled structure with a narrow channel of 2 mm in width to form high-speed mercury flow ($$sim$$4m/s) has been adopted. After operation, the beam window was cut to inspect the effect of the cavitation damage mitigation on inner wall. We optimized cutting conditions through the cold cutting tests, succeeding in cutting the target No.2 (without damage mitigation technologies) smoothly in 2017, and target No.8 with damage mitigation technologies. In the workshop, progress of cavitation damage observation for the target vessel will be presented.

Journal Articles

New design and fabrication technology applied in mercury target vessel #8 of J-PARC

Wakui, Takashi; Wakai, Eiichi; Kogawa, Hiroyuki; Naoe, Takashi; Hanano, Kohei; Haga, Katsuhiro; Takada, Hiroshi; Shimada, Tsubasa*; Kanomata, Kenichi*

JPS Conference Proceedings (Internet), 28, p.081002_1 - 081002_6, 2020/02

A mercury target vessel of J-PRAC is designed with a triple-walled structure consisting of the mercury vessel and a double-walled water shroud with internal and external vessels. During the beam operation at 500 kW in 2015, small water leakages from a water shroud of the mercury target vessel occurred twice. Design, fabrication and inspection processes were improved based on the lessons learned from the target failures. The total length of welding lines at the front of the mercury target vessel decreases drastically to approximately 55% by adopting monolithic structure cut out from a block of stainless steel by the wire-electrical discharge machining. Thorough testing of welds by radiographic testing and ultrasonic testing was conducted. The fabrication of the mercury target vessel #8 was finished on September 2017 and the beam operation using it started. Stable beam operation at 500 kW has been achieved and it could experience the maximum beam power of 1 MW during a beam test.

Journal Articles

Study on fracture behaviour of through-wall cracked elbow under displacement control load

Machida, Hideo*; Koizumi, Yu*; Wakai, Takashi; Takahashi, Koji*

Nippon Kikai Gakkai M&M 2019 Zairyo Rikigaku Kanfarensu Koen Rombunshu (Internet), p.OS1307_1 - OS1307_5, 2019/11

This paper describes the fracture test and fracture analysis of a pipe under displacement control load. In order to grasp the fracture behavior of the circumferential through-wall cracked pipe, which is important in evaluating the feasibility of leak before break (LBB) in sodium cooled reactor piping, a fracture test in case of a circumferential throughwall crack in the weld line between an elbow and a straight pipe was carried out. From this test, it was found that no pipe fracture occurs in the displacement control loading condition even if a large circumferential through-wall crack (180$$^{circ}$$) was assumed. The fracture analysis of the pipe was carried out using Gurson's parameters set based on the tensile test results of the tested pipe material. The analytic results agree well with the test results, and it was found that it will be possible to predict the fracture behavior of sodium cooled reactor piping.

Journal Articles

A Proposal of inelastic constitutive equations of lead alloys used for structural tests simulating severe accident conditions

Hashidate, Ryuta; Onizawa, Takashi; Wakai, Takashi; Kasahara, Naoto*

Proceedings of 2019 ASME Pressure Vessels and Piping Conference (PVP 2019) (Internet), 10 Pages, 2019/07

Under the severe accident conditions, structural materials of nuclear power plants are subjected to excessive high temperature. Although it is very essential to clarify how the structure collapses under the severe accident conditions, the failure mechanisms in such high temperatures are not clarified. However, it is very difficult and expensive to perform structural tests using actual structural materials. Therefore, we propose to use lead alloys instead of actual structural materials. Because the strength of lead alloys is much poorer than that of the actual structural materials, failure can be observed at low temperature and by small load. For demonstration of analogy between the failure mechanisms of lead alloys structure at low temperature and those of the actual structures at high temperature, numerical analyses are required. So, we confirm the material characteristics of lead alloys and develop inelastic constitutive equations of lead alloy required for finite element analyses.

Journal Articles

Optimum temperature for HIP bonding invar alloy and stainless steel

Wakui, Takashi; Ishii, Hideaki*; Naoe, Takashi; Kogawa, Hiroyuki; Haga, Katsuhiro; Wakai, Eiichi; Takada, Hiroshi; Futakawa, Masatoshi

Materials Transactions, 60(6), p.1026 - 1033, 2019/06

 Times Cited Count:0 Percentile:100(Materials Science, Multidisciplinary)

The mercury target has large size as 1.3$$times$$1.3$$times$$2.5 m$$^{3}$$. In view of reducing the amount of wastes, we studied the structure so that the fore part could be separated. The flange is required to have high seal performance less than 1$$times$$10$$^{-6}$$ Pa m$$^{3}$$/s. Invar with low thermal expansion is a candidate. Due to its low stiffness, however, the flange may deform when it is fastened by bolts. Practically invar is reinforced with stainless steel where all interface between them has to be bonded completely with the HIP bonding. In this study, we made specimens at four temperatures and conducted tensile tests. The specimen bonded at 973 K had little diffusion layer, and so fractured at the interface. The tensile strength reduced with increasing the temperature, and the reduced amount was about 10% at 1473 K. The analyzed residual stresses near the interface increased by 50% at maximum. Then, we concluded that the optimum temperature was 1173 K.

Journal Articles

Effect of local plastic component on crack opening displacement and on J-integral of a circumferential penetrated crack

Machida, Hideo*; Arakawa, Manabu*; Wakai, Takashi

Proceedings of 27th International Conference on Nuclear Engineering (ICONE-27) (Internet), 8 Pages, 2019/05

This paper describes the effect of local plastic component on J-integral and crack opening displacement (COD) evaluation of a circumferential penetrated crack, applicable to the leak before break (LBB) assessment for sodium cooled fast reactor (SFR) components. J-integral COD evaluation methods are generally formulated as a summation of elastic and plastic components, and so far many evaluation formulae based on these two components have been proposed. However, strictly, the plastic component consists of local plastic and fully plastic components. Many of the conventional evaluation methods often consider only the fully plastic component as the plastic component. The reason for this is that the effect of the local plastic component is much smaller than that of the fully plastic component excluding materials with extremely small work hardening. In contrast, for materials with high yield stress and small work hardening, such as modified 9Cr-1Mo steel which is one of the candidate materials for SFR piping, the effect of the local plastic component on J-integral and COD cannot be ignored. Therefore, the authors propose formulae taking the effect of local plastic component on J-integral and COD into account, based on finite element analysis (FEA) results, so that it is easy to apply to crack evaluation. The formulae will be employed in the guidelines on LBB assessment for SFR components published from Japan Society of Mechanical Engineers (JSME).

Journal Articles

Study on creep damage assessment method for Mod.9Cr-1Mo steel by sampling creep testing with thin plate specimen

Kanayama, Hideyuki*; Hiyoshi, Noritake*; Ogawa, Fumio*; Kawabata, Mie*; Ito, Takamoto*; Wakai, Takashi

Zairyo, 68(5), p.421 - 428, 2019/05

This study presents creep damage assessment method for Mod. 9Cr-1Mo steel by sampling creep testing with thin plate specimen. Tensile creep rupture tests were performed using three different sizes of specimen under two different test environments to verify the creep testing with the thin plate specimen. Time to rupture of Mod. 9Cr-1Mo steel using three different sizes were almost same. In addition, there was no effect of environment on time to rupture. Pre-damaged thin plate specimens were machined from a bulk specimen's gage section that pre-damage test was performed with. Pre-damage based on life fraction rule were 8%, 16% and 25%. No effect of the process of machining pre-damaged specimen on time to rupture was confirmed by verification tests in same test condition as pre-damage test. Stress acceleration creep rupture tests were performed to estimate creep damage assessment. Creep damage assessment by stress acceleration creep rupture tests was sufficiently accurate estimate. Creep damage assessments by Vickers hardness and lath width were compared with the assessment by stress acceleration creep rupture tests to study applicability of these methods.

Journal Articles

Ultrasonic inspection technique for weld part of mercury target vessel for spallation neutron source

Wakui, Takashi; Wakai, Eiichi; Naoe, Takashi; Kogawa, Hiroyuki; Haga, Katsuhiro; Takada, Hiroshi; Shintaku, Yohei*; Li, T.*; Kanomata, Kenichi*

Choompa Techno, 30(5), p.16 - 20, 2018/10

A mercury target vessel has been used for the spallation neutron source at J-PARC. It has a complicated multi-layered structure composed of a mercury target and a surrounding double-walled water shroud, which is assembled with thin plates (minimum thickness of 3 mm) by welding. Thus, welding inspection during the manufacturing process is important. We investigated the applicability of new ultrasonic inspections using specimens (thickness of 3 mm) with defects to improve the accuracy of welding inspection for the mercury target vessel. Immersion ultrasonic testing using a probe (frequency of 50 MHz) could detect a spherical defect with a diameter of 0.2 mm. The size was smaller than target value of 0.4 mm. The length of unwelded region estimated using the phased array ultrasonic testing corresponded with the actual length (0.8 - 1.5 mm).

Journal Articles

Improvement of penetrate crack length evaluation method for LBB assessment of sodium-cooled fast reactor components

Wakai, Takashi; Machida, Hideo*; Arakawa, Manabu*

Nippon Kikai Gakkai 2018-Nendo Nenji Taikai Koen Rombunshu (DVD-ROM), 5 Pages, 2018/09

According to the fitness for service code of Sodium-Cooled fast Reactor (SFR), the volumetric tests as in-service inspection can be replaced with continuous leak monitoring, where the Leak Before Break (LBB) is demonstrated, because the primary stress caused by internal pressure is not significant in SFR components. Basically, if the detectable crack length and the penetrated crack length are sufficiently smaller than the unstable critical crack length, it can be concluded that LBB is successfully demonstrated. The authors had already proposed a simplified method to calculate the penetrated crack length both of the circumferential and axial cracks in the pipe as a function of pipe geometry, fatigue crack growth characteristics and loading conditions. However, some problems in the method have been pointed out in the process of the reviewing by the JSME code committee. This study describes an improved method to calculate the penetrated crack length.

Journal Articles

Effect of welding on gigacycle fatigue strength of austenitic stainless steels

Naoe, Takashi; Wakui, Takashi; Kogawa, Hiroyuki; Wakai, Eiichi; Haga, Katsuhiro; Takada, Hiroshi

Advanced Experimental Mechanics, 3, p.123 - 128, 2018/08

A mercury target vessel, composed of SUS316L, is used for the pulsed neutron source and is assembled via TIG welding. While in operation, the target vessel suffers ca. 10$$^9$$ loading cycles with a high strain rate of ca. 50 s$$^{-1}$$ because of the proton-beam-induced pressure waves in mercury. The gigacycle fatigue strength for solution annealed SUS316L stainless steels and its welded specimens were investigated through ultrasonic fatigue tests. The experimental results showed that an obvious fatigue limit was not observed at fewer than 10$$^9$$ cycles for the base metal. In the case of no weld defects observed via penetration tests, the fatigue strength of the removed-weld-bead specimen, in which the weld lines were arranged at the center of the specimen, appeared to be slightly higher than that of the base metal. By contrast, as-welded specimens with the weld bead intact showed apparent degradation of the fatigue strength owing to the stress concentration around the weld toe.

Journal Articles

Recent studies for structural integrity evaluation and defect inspection of J-PARC spallation neutron source target vessel

Wakui, Takashi; Wakai, Eiichi; Naoe, Takashi; Shintaku, Yohei*; Li, T.*; Murakami, Kazuya*; Kanomata, Kenichi*; Kogawa, Hiroyuki; Haga, Katsuhiro; Takada, Hiroshi; et al.

Journal of Nuclear Materials, 506, p.3 - 11, 2018/08

 Times Cited Count:0 Percentile:100(Materials Science, Multidisciplinary)

The mercury target vessel is designed as multi-walled structure with thin wall (min. 3 mm), and assembled by welding. In order to estimate the structural integrity of the vessel, it is important to measure the defects in welding accurately. For nondestructive tests of the welding, radiographic testing is applicable but it is difficult to detect for some defect shapes. Therefore it is effective to do ultrasonic testing together with it. Because ultrasonic methods prescribed in JIS inspect on the plate with more than 6 mm in thickness, these methods couldn't be applied as the inspection on the vessel with thin walls. In order to develop effective method, we carried out measurements using some testing method on samples with small defect whose size is specified. In the case of the latest phased array method, measured value agreed with actual size. It was found that this method was applicable to detect defects in the thin-walled structure for which accurate inspection was difficult so far.

Journal Articles

Development of a crack opening displacement assessment procedure considering change of compliance at a crack part in thin wall pipes made of modified 9Cr-1Mo steel

Wakai, Takashi; Machida, Hideo*; Arakawa, Manabu*; Yanagihara, Seiji*; Suzuki, Ryosuke*; Matsubara, Masaaki*

Proceedings of 26th International Conference on Nuclear Engineering (ICONE-26) (Internet), 9 Pages, 2018/07

This paper studies crack opening displacement (COD) evaluation methods used in Leak-Before-Break (LBB) assessment of Sodium cooled Fast Reactor (SFR) pipe. For SFR pipe, the continuous leak monitoring will be adopted as an alternative to a volumetric test of the weld joints under conditions that satisfy LBB. The sodium pipes are made of ASME Gr.91 (modified 9Cr-1Mo steel). Thickness of the pipes is small, because the internal pressure is very small. Modified 9Cr-1Mo steel has a relatively large yield stress and small work hardening coefficient comparing to the austenitic stainless steels which are currently used in the conventional plants. In order to assess the LBB behavior of the sodium pipes made of modified 9Cr-1Mo steel, the coolant leak rate from a through wall crack must be estimated properly. Since the leak rate is strongly related to the crack opening displacement (COD), an appropriate COD assessment method must be established to perform LBB assessment. However, COD assessment method applicable for SFR pipes - having thin wall thickness and made of small work hardening material - has not been proposed yet. Thus, a COD assessment method applicable to such a pipe was proposed in this study. In this method, COD was calculated by classifying the components of COD; elastic, local plastic and fully plastic. In addition, the verification of this method was performed by comparing with the results of a series of four-point bending tests using modified 9Cr-1Mo steel pipe having a circumferential through wall notch. As a result, in some cases, COD were over-estimated especially for large cracks. Although the elastic component of COD is still over-estimated for large cracks, leak evaluation from small cracks is much more important in LBB assessment. Therefore, this study recommends that only the elastic component of COD should be adopted in LBB assessment of SFR pipes.

Journal Articles

Proposal of simplified J-integral evaluation method for a through wall crack in SFR pipe made of Mod.9Cr-1Mo steel

Wakai, Takashi; Machida, Hideo*; Arakawa, Manabu*; Kikuchi, Koichi*

Proceedings of ASME Symposium on Elevated Temperature Applications of Materials for Fossil, Nuclear, and Petrochemical Industries, 7 Pages, 2018/04

A simplified J-integral evaluation method applicable to unstable failure analysis in Leak Before Break (LBB) assessment of Sodium-cooled Fast Reactor (SFR) in Japan was proposed. Mod.9Cr-1Mo steel is supposed to be a candidate material for the coolant systems of SFR in Japan. This steel has relatively high yield strength and poor fracture toughness comparing to those of conventional austenitic stainless steels. In addition, SFR pipe has small thickness and large diameter. As a J-integral evaluation method for circumferential through-wall crack in a cylinder, EPRI has proposed a fully plastic solution method. However, the geometry of SFR pipe and material characteristics of Mod.9Cr-1Mo steel exceed the applicable range of EPRI's method. Therefore, a series of elastic, elasto-plastic and plastic finite element analyses (FEA) were performed for a pipe with a circumferential through-wall crack to propose a J-integral evaluation method applicable to such loading conditions. J-integrals obtained from the FEA were resolved into elastic, local plastic and fully plastic components. Each component was expressed as a function of analytical parameter, such as pipe geometries, crack size, material characteristics and so on. As a result, a simplified J-integral evaluation method was proposed. The method enables to conduct 2 parameter failure analysis using J-integral without any fracture mechanics knowledge.

Journal Articles

Proposal on LBB evaluation conditions for sodium cooled fast reactor pipes and effects of pipe parameters

Yada, Hiroki; Takaya, Shigeru; Wakai, Takashi; Nakai, Satoru; Machida, Hideo*

Nippon Kikai Gakkai Rombunshu (Internet), 84(859), p.17-00389_1 - 17-00389_15, 2018/03

no abstracts in English

Journal Articles

Study on a unified criterion for preventing plastic strain accumulation due to long distance travel of temperature distribution

Okajima, Satoshi; Wakai, Takashi; Kawasaki, Nobuchika

Mechanical Engineering Journal (Internet), 4(5), p.16-00641_1 - 16-00641_11, 2017/10

Journal Articles

Study of the calculation method for the elastic follow-up coefficient by inelastic analysis

Watanabe, Sota*; Kubo, Koji*; Okajima, Satoshi; Wakai, Takashi

Nippon Kikai Gakkai M&M 2017 Zairyo Rikigaku Kanfarensu Koen Rombunshu (Internet), p.581 - 585, 2017/10

no abstracts in English

Journal Articles

Study on the seismic buckling evaluation method for Mod. 9Cr-1Mo steel cylindrical vessel

Okafuji, Takashi*; Miura, Kazuhiro*; Sago, Hiromi*; Murakami, Hisatomo*; Kubo, Koji*; Sato, Kenichiro*; Wakai, Takashi; Shimomura, Kenta

Nippon Kikai Gakkai M&M 2017 Zairyo Rikigaku Kanfarensu Koen Rombunshu (Internet), p.591 - 595, 2017/10

no abstracts in English

Journal Articles

Saturation behavior of thermal ratcheting strain due to long range travel of temperature distribution

Okajima, Satoshi; Wakai, Takashi

Nippon Kikai Gakkai 2017-Nendo Nenji Taikai Koen Rombunshu (DVD-ROM), 5 Pages, 2017/09

It was reported that the long distance travel of temperature distribution causes a new type of thermal ratcheting, even in the absence of primary stress. In this paper, based on the results of inelastic finite element analyses, we investigated saturation behavior of thermal ratcheting strain due to long range travel of temperature distribution. As a result, we revealed that the long distance travel of temperature distribution generates plastic strain distribution made maximum at the central part. Because of the shape of the generated strain distribution, the residual stress accumulates even at the central part of the region passed through the temperature distribution. In the case with excessive long traveling of temperature distribution, the region with plastic deformation extended to the surrounding region. Otherwise, sufficient magnitude of residual stress to cause shakedown behavior accumulated on entire region, and the accumulation of the plastic strain saturated.

Journal Articles

Thermal fatigue test on dissimilar welded joint between Gr.91 and 304SS

Wakai, Takashi; Kobayashi, Sumio; Kato, Shoichi; Ando, Masanori; Takasho, Hideki*

Proceedings of 25th International Conference on Nuclear Engineering (ICONE-25) (CD-ROM), 7 Pages, 2017/07

This paper describes a thermal fatigue test on a structural model with a dissimilar welded joint. In the present design of JSFR, there may be dissimilar welded joints between ferritic and austenitic steels especially in IHX and SG. Creep-fatigue is one of the most important failure modes in JSFR components. However, the creep-fatigue damage evaluation method has not been established for dissimilar welded joint. To investigate the evaluation method, structural test will be needed for verification. Therefore, a thermal fatigue test on a thick-wall cylinder with a circumferential dissimilar welded joint between Mod.9Cr-1Mo steel and 304SS was performed. Since the coefficients of thermal expansion of these steels were significantly different, buttering layer of Ni base alloy was installed between them. After the completion of the test, deep cracks were observed at the HAZ in 304SS, as well as at HAZ in Mod.9Cr-1Mo steel. There were many tiny surface cracks in BM of 304SS. According to the fatigue damage evaluation based on the finite element analysis results, the largest fatigue damage was calculated at HAZ in 304SS. Large fatigue damage was also estimated at BM of 304SS. Fatigue cracks were observed at HAZ and BM of 304SS in the test, so that analytical results are in a good agreement with the observations. However, though relatively small fatigue damage was estimated at HAZ in Mod.9Cr-1Mo steel, deep fatigue cracks were observed in the test. To identify the cause of such a discrepancy between the test and calculations, we performed a series of finite element analyses. Some metallurgical investigations were also performed.

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